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Concept: MNDO-based semi-empirical methods in quantum chemistry have found widespread application in the modelling of large and complex systems. A method for the analytic evaluation of first and second derivatives of molecular properties against semi-empirical parameters in MNDO-based NDDO-descendant models is presented, and the resultant parameter Hessian is compared against the approximant currently used in parameterization for the PMx models. Methods: As a proof of concept, the exact parameter Hessian is employed in a limited reparameterization of MNDO for the elements C, H, N, O and F using 1206 molecules for reference data (heats of formation, ionization energies, dipole moments and reference geometries). The correctness of our MNDO implementation was verified by comparing the calculated molecular properties with the MOPAC program. [ABSTRACT FROM AUTHOR]

In this work, we performed a quantum-chemical calculation of a graphene oxide molecule by the MNDO method. An optimised geometric and electronic structure of this compound is obtained. It was found that the studied molecule, graphene oxide, belongs to the class of very weak Brönsted acids (pKa > 14). [ABSTRACT FROM AUTHOR]

The first quantum-chemical calculation of the molecules 22, 124 and 200 hexagons by the method MNDO was performed with optimisation of the geometry for all parameters by a standard gradient method. The optimised geometric and electronic structures of 22, 124 and 200 hexagons were obtained and their acid strength was theoretically evaluated (pKa = 33). It was found that the molecules belong to the class of very weak CH-acids (pKa > 14). [ABSTRACT FROM AUTHOR]

Polychlorinated biphenyls (PCBs) are chlorinated organic compounds and well known carcinogenic and toxic pollutants. Currently, their detection and degradation to products with less risk are among environmental and health priorities. Passing 2,2′,4,4′,5,5′-Hexachlorobiphenyl (PCB-153) through the armchair single-walled carbon nanotubes (SWCNTs) (8, 8) and (10, 10) was investigated by Modified Neglect of Diatomic Overlap in the semi-empirical method. The analysis of results suggests that there are meaningful changes in the middle of the tubes. Based on the obtained evidence, the nanotubes have substantial potential to interact with the PCB-153 molecule effectively. The results show that the increased diameter in the armchair SWCNTs improves the detection and degradation potential of the tube to PCBs. According to the calculated thermodynamic parameters, the diameter of nano-structures is an effective factor in PCBs removal efficiency, as it could be helpful to make a more sensitive PCB nano-sensor. [ABSTRACT FROM AUTHOR]

This study aims to determine the spectral properties of the trimethyl Aluminum molecule (C3H9Al) using the Semi-empirical quantum programs [HyperChem8.0, WinMopac7.21] by (MNDO-PM3) (Modified Neglect of Differential Overlap-Parameterization Model3). This method characterized by the flexibility, the simplicity and the quick performance. The equilibrium geometry and the potential curve were calculated and then from the curve, the total energy of the molecule at the equilibrium distance was calculated. In addition, the dissociation energy, the energy of electron , the core-core repulsion, the ionization energy, the zero point energy, the energy gap, the frequencies and basic vibrational modes have been calculated. The study’s results were in agreement with the previous research.

We assumed that in our earlier papers that the interaction between some metal atoms and their intercalation inside the nanotube is possible. These facts suggest us that the boron-carbon nanotube may be the new material, which is as well as pure carbon nanotube become the basis of the nanoelelctronic devices. For this reason the continuation of metal adsorption mechanism studying is important. The way of intercalation (doping) of alkaline metal inside the BC3-nanotube and modeling of the regular external adsorption is suggested. Results of researches showed that doping of boron-carbon nanotubes is possible and the electron density is distributed near nanotube surface atoms. Regular external adsorption of the alkaline metal atoms causes decreasing of the energy gap compared with the pure boron-carbon nanotube. So, we can obtain that intercalation of metal atom inside the BC

The reaction mechanism of the oxidation of 1,1-dimethylhydrazine by iodine was examined using semi-empirical and density functional theory methods. The oxidation proceeded via an independent pathway which was monitored and the results of the kinetic and thermodynamic study were determined. It was found that the reactant as seen by their determined binding energy would readily undergo a two-step reaction leading to the decomposition of the reactants and the formation of more stable products. [ABSTRACT FROM AUTHOR]

An improved procedure for the "one-pot" preparation of cathecols from methylenedioxy-ring cleavage reaction of safrole derivatives with aluminum chloride is described. In substrates substituted by conjugated electron-withdrawing groups (carboxyaldehyde or nitro groups) the regioselective formation of the easily isolable chloromethyl ether intermediates was observed. From these intermediates the syntheses of mono-O-methylated phenols (3-hydroxy-4-methoxybenzaldehyde, 2-bromo-4-methoxy-5-hydroxybenzaldehyde, 2-nitro-4-methoxy-5-hydroxybenzaldehyde and 2-methoxy-4-(2-oxoprop-1-yl)-5-nitrophenol) were accomplished. Based on these experimental data and semi-empirical (MNDO) molecular orbital calculations, a mechanistic rationale that explains the observed regioselectivities was also proposed.

In this study, heat energy, atomic charges and dipole moments permit qualitative predictions about the substituent effects by a chlorine or methyl group on the benzodiazepine properties in the ground state. AM1 and MNDO semi-empirical methods of calculation are used to obtain information on the structural and energy properties for some benzodiazepine derivatives. [ABSTRACT FROM AUTHOR]

The newly implemented quantum-chemical/molecular-mechanical (QM/MM) functionality of the Groningen molecular simulation (GROMOS) software for (bio)molecular simulation is described. The implementation scheme is based on direct coupling of the GROMOS C++ software to executables of the quantum-chemical program packages MNDO and TURBOMOLE, allowing for an independent further development of these packages. The new functions are validated for different test systems using program and model testing techniques. The effect of truncating the QM/MM electrostatic interactions at various QM/MM cutoff radii is discussed and the application of semiempirical versus density-functional Hamiltonians for a solute molecule in aqueous solution is compared. © 2012 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

The electronic structures of 3-acetyl-2-methylbenzothiophene, the cations resulting from its C-protonation at positions 4-7 and O-protonation, and the dications formed upon protonation at positions 4-7 of the O-protonation product were studied by the MNDO, HF/3-21G, and B3LYP/3-21G methods. Analysis of the relative energies of these cations and dications is in line with the earlier authors' data on the predominant reactivity of positions 4 and 6 of the 3-acetyl-2-methylbenzothiophene molecule toward acylation. [ABSTRACT FROM AUTHOR]

Epoxide ring openings play important roles in a variety of chemical situations and are believed to be important in the metabolic processes through which some polycyclic aromatic hydrocarbons are converted to carcinogenic species. In this report, the energetics and directionalities of acid-catalyzed epoxide ring openings in benzene and naphthalene are examined using quantum chemical (MNDO/d and MP2/6-31G*) methods. In particular, the consequences of local saturations in the aromatic systems are examined and shown to exert strong influences on the resulting energetics and product distributions. Partial ring saturation in an aromatic system leads to barrier-free, spontaneous ring opening of the protonated epoxide in a favored direction and eliminates opening in the opposite direction. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [ABSTRACT FROM AUTHOR]

Abstract: In this work we report on a vibrational spectroscopic study on the structural aspects of the binary xCaO·(100− x)SiO2, xMgO·(100− x)SiO2 and ternary xCaO·yMgO (100− x − y)SiO2 systems for various x values up to 50%. Raman and IR spectroscopies have revealed the degradation of the silica network with the addition of the oxide modifiers. The main differences in the spectra of the alkaline earth modified sol–gel glasses are located in the region 900–1200cm−1 where vibrations of SiO4 tetrahedra with a variable number of non-bridging oxygen atoms are located. A comparison between the structural modifications of bulk (melt-quenched) and porous glasses is also advanced revealing particular differences in the network modification of the two types of glasses. CaO is more drastic than MgO in depolymerizing silica''s network and both of them cause more significant structure modifications in the sol–gel glasses. The depolymerization structural units offer the appropriate sites in the glass network that are responsible for the facilitation of Ca/Mg dissolution, which is an important step for bioactivity. Furthermore, to support the conclusions drawn from the experimental data, preliminary molecular structure simulations using semiempirical molecular orbital theory are also presented. [Copyright &y& Elsevier]

This work deals with the molecular orbital calculation studies performed on different diazadienes to assess their reactivity pattern. The interaction of these diazadienes with various electron-poor and electron-rich dienophiles leads to the formation of diazines and tetrazines as the cycloadducts. The results from frontier orbital interactions were used to rationalize the reactivity and predictability of NDAC and IEDDAC reaction pathways. Correlation studies were also performed to predict reactivity sequence using a number of electronic descriptors, such as electrophilicity index (ω), chemical potential (µ), electronic charge ΔNmax, and chemical hardness η. Moreover, these studies exhibit good compatibility with experimental observations. [ABSTRACT FROM AUTHOR]

Abstract: Calculations on the molecular structure of three natural brassinosteroids using several semiempirical Hamiltonians: MNDO, AM1, PM3, PDDG/MNDO, PDDG/PM3 have been done to test their accuracy for predicting structural features of these molecules when compared with available crystallographic data. The best molecular structures were obtained by AM1 and PDDG/PM3 semiempirical Hamiltonians. The worst performances were observed for MNDO and PDDG/MNDO. Molecular orbitals (i.e. highest occupied molecular orbital (HOMO) and lower unoccupied molecular orbital (LUMO)), electrostatic potential maps, and atomic charges with all semiempirical methods were also calculated. Results showed that the HOMO was always located in the lateral chain and the LUMO in the lactone group of ring B for the three systems analyzed. Lactone group elicited higher electrostatic potentials than hydroxyl groups. Analysis of charges also revealed an accumulation of charges in the lactone group and in the lateral chain. These modelling results explain how the biological activity process of these molecules occurs when there is electron exchange. The results also indicate that the interaction of brassinosteroids with receptors could involve specific areas of the molecule. [Copyright &y& Elsevier]

Abstract: In order to investigate the molecular geometry, structural analysis of the title compound (4a) was carried out by single crystal X-ray diffraction. Tautomeric properties and conformations were investigated by semi-empirical (AM1) and ab initio quantum mechanical (RHF/6-31G) calculations. The compound prefers thione tautomer in the solid state. On the other hand, semiempirical calculations show that in the gas phase the thiol tautomer is energetically favored in most cases. [Copyright &y& Elsevier]

Hückel type cyclacenes having R = 3–10 have been subjected to quantum chemical analyses at the level of AM1, MNDO and MINDO/3 (UHF and RHF) type semi-empirical treatments. The structures have been found to be stable but endothermic in nature. No cryptoannulenic effect has been observed when AM1 (UHF) and MNDO (UHF) treatments are used. Whereas MINDO/3 (UHF) and all the RHF type calculations indicated certain degree of cryptoannulenic effect R = 4 through 8 for the heat of formation values. Frontier molecular orbital energies of the systems have also been reported. [ABSTRACT FROM AUTHOR]

Abstract: The corrosion inhibition efficiencies of some triazole, oxadiazole and thiadiazole derivatives for steel in presence of acidic medium have been studied by using AM1, PM3, MINDO/3 and MNDO semi-empirical SCF molecular orbital methods. Geometric structures, total negative charge on the molecule (TNC), highest occupied molecular energy level (E HOMO), lowest unoccupied molecular energy level (E LUMO), core–core repulsion (CCR), dipole moment (μ) and linear solvation energy terms, molecular volume (V i) and dipolar-polarization (π *), were correlated to corrosion inhibition efficiency. Four equations were proposed to calculate corrosion inhibition efficiency. The agreement with the experimental data was found to be satisfactory; the standard deviations between the calculated and experimental results ranged between ±0.03 and ±4.18. The inhibition efficiency was closely related to orbital energies (E HOMO and E LUMO) and μ. The correlation between quantum parameters and experimental inhibition efficiency has been validated by single point calculations for the semi-empirical AM1 structures using B3LYP/6-31G** as a higher level of theory. The proposed equations were applied to predict the corrosion inhibition efficiency of some related structures to select molecules of possible activity from a presumable library of compounds. [Copyright &y& Elsevier]

Thegroup functions technique is a natural way to introduce local description into quantum chemistry. It can also be a basis for construction of numerically effective computational schemes having almost linear growth of computational costs with that of the size of the system. Previously, we constructed a family of computationally efficient semiempirical methods based on the variationally determined strictly local geminals (SLGs). It was implemented with four popular parameterization schemes (MINDO/3, MNDO, AM1, and PM3). Because of construction details, its applicability was restricted only to compounds with well-defined two-electron two-center chemical bonds and lone pairs. We generalize the previous treatment to make the electronic structure calculations possible for a wider class of compounds without loss of computational efficiency. The proposed scheme (SLG/SCF) is based on the general group function approach combining different descriptions for different electron groups: essentially local two-electron ones are described by geminals, while those with other numbers of electrons are described in the one-electron approximation. We implement the RHF, UHF, and ROHF approaches for the groups with delocalized electrons. This approach is tested for a series of radicals and molecules with extended π-electron systems. It is shown that the SLG/SCF-based methods describe the experimental data not worse than the corresponding SCF procedures and provide a good starting point for calculations of polyatomic molecular systems. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [ABSTRACT FROM AUTHOR]

Abstract: Quantum chemical calculations were performed on azacyclo :glyph name="sbnd" />C14 amines, open chain :glyph name="sbnd" />C14 amines and phenylazacyclo :glyph name="sbnd" />C14 amines. Inspection of the calculated parameters and corrosion inhibition efficiencies were made to observe any clear links, which might exist between the two. Possible correlations between experimental inhibition efficiencies and parameters such as dipole moment (μ), highest occupied (E HOMO) and lowest unoccupied (E LUMO) molecular orbitals and the differences between them, HOMO–LUMO gap (Δ), as well as some structural characteristics were investigated. The models of the inhibitors were optimized with the Modified Neglect of Diatomic Overlap (MNDO) method. The Quantitative Structure Activity Relationship (QSAR) approach has been used and a composite index of some quantum chemical parameters were constructed in order to characterize the inhibition performance of the tested molecules. The inhibition effect of polymethylene amines is closely related to orbital energies and/or energy gap and dipole moment. [Copyright &y& Elsevier]

We describe the syntheses, characterization and theoretical investigation of two new monoazo Schiff-base ligands 5-[(4-methylphenyl)azo]- N -(2′-hydroxyphenyl)salicylaldimine (SB1) and 5-[(4-nitrophenyl)azo]- N -(2′-hydroxyphenyl)salicylaldimine (SB2). AM1 semiempirical calculations were used to study the structures, electronic properties and second-order nonlinearity of these compounds. Theoretical calculations are useful for describing the relationship between the electronic structure of molecular systems and their nonlinear optical (NLO) response. Second-order NLO calculations and comparison of the NLO properties of these ligands with others reveal that the presence of an NO 2 group enhances the second-order NLO properties of the ligands. [ABSTRACT FROM AUTHOR]

A systematic investigation on the molecular behaviors of all the possible isomers of C34B2 and C34N2 formed from the initial C36 fullerene with C6v and C6h symmetries have been performed using the semi-empirical AM1 and MNDO methods. The equilibrium geometrical structures, heats of formation, HOMO–LUMO gap energies, heats of atomization, ionization potentials (IP) and affinity potentials (AP), conjugate effect and deformation energies of C34X2 (X=N, B) have been studied. The calculation results show that the heterofullerenes are less stable than C36, and the C34N2 should be more stable than the boron analog C34B2. Both empirical methods in this work indicate that the most stable isomer of C34X2 (X=N, B) corresponds to 1,4-substitution in the cyclohexatriene unit which locating at the equatorial belt of the C36 cage. Generally speaking, the C34X2 (X=N, B) isomers with the doped atoms near the equatorial belt are more stable that the rest. The heterofullerenes C34X2 have bigger IP and smaller AP compared with C36, thus the redox activity of C36 can be reduced by doping. The results of π-orbital axis vector analysis show that replacements of carbon atoms with either nitrogen or boron can notably release the strains in local part of the cage. Both C34N2 and C34B2 are expected to have significantly different chemical and physical properties from those of the fullerenes. [Copyright &y& Elsevier]

The cyclotriphosphate hexahydrate of manganese and silver, MnAg 4 (P 3 O 9 ) 2 .6H 2 O, was prepared by Boullé's process. MnAg 4 (P 3 O 9 ) 2 .6H 2 O crystallizes in the triclinic system, P-1, Z = 1 and its structure was determined by Rietveld's method. The refinement of 54 parameters by this method, using 1752 independent reflections leads to Rw p = 0.098, R p = 0.065 and R B = 0.033 on the basis of its isotype NiAg 4 (P 3 O 9 ) 2 .6H 2 O. The unit cell parameters of this salt are the following a = 8.824(1)Å, b = 8.485(1)Å, c = 6.609(1)Å, α = 90.30(1)°, β = 92.89(1)° and γ = 107.28(1)°. The thermal behavior of this new compound was studied, between 25 and 600°C, under atmospheric pressure by infrared spectrometry, X-ray diffraction, thermal analyses TGA and DTA coupled. This study allows us the identification and the crystallographic characterization of a new phase, MnAg 2 (PO 3 ) 4 isotype of ZnRb 2 (PO 3 ) 4 , obtained between 350 and 450°C, mixed with the long-chain polyphosphate of silver AgPO 3 . The kinetic characteristics of the dehydration of MnAg 4 (P 3 O 9 ) 2 .6H 2 O and the thermal phenomena accompanying this dehydration were determined and discussed on the basis of the proposed crystalline structure. The vibrational spectrum of the, MnAg 4 (P 3 O 9 ) 2 .6H 2 O, was examined and interpreted in the domain of the stretching vibrations of the P 3 O 9 rings, on the basis of the proposed crystalline structure and in the light of the calculation of the thirty fundamental IR frequencies for the idealized D 3h symmetry. [ABSTRACT FROM AUTHOR]

A systematic calculation of the potential curves or surfaces for 1,2-shift has been realized by using MNDO or other models in MOPAC programs. By referring to the previous author’ viewpoints, the 1,2-shift can be divided into two categories. 1,2-electron-deficient shift is that the electronic configuration of the atom which accepts the migrating group is a cation or an electron-deficient atom, and 1,2-anion shift is the one that the accepted atom of the migration group is a negative ion. In terms of the experimental facts and the calculation of the potential surfaces, in electron-deficient shift such as Beckmann or Baeyer-Villiger rearrangement, the migration occurs through a transition complex formed between the π-bond and the cation or electron-deficient migrating group, but in anion shift such as Wittig or Stevens rearrangement, the electron pair in π-orbit excites at first to π orbit, and then the migration occurs through the new formed complex between the anion migration group and the vacant π orbit. The above mechanisms explain reasonably the intramolecular properties, the configuration retentions of the migration group, and the corresponding migratory aptitudes of the two type 1, 2-shifts. The partial and less important free radical reaction of 1, 2-anion shift has been explained by the π-complex mechanism too. [ABSTRACT FROM AUTHOR]

Two quasi-multipole electrostatic models for molecular charge distributions are presented. They assign arrays of point charges to nonhydrogen atoms on the basis of hybrid orbitals or localised molecular orbitals. When used with common semiempirical MO-techniques, they reproduce natural atomic orbital derived point charge (NAO-PC) and ab initio molecular potentials well. The localised orbital technique (LMO-PC) is intuitively more attractive than the hybrid orbital-point charge (HO-PC) method, although the former is more CPU-intensive. [ABSTRACT FROM AUTHOR]